In recent years, there have been proposed and developed various variable valve actuation systems capable of variably adjusting an engine valve timing depending on operating conditions of an internal combustion engine. One such variable valve actuation system has been disclosed in Japanese Patent Provisional Publication No. 10-227236 (hereinafter is referred to as “JP10-227236”). The variable valve actuation system disclosed in JP10-227236 is comprised of a so-called rotary vane type valve timing control (VTC) system. In such a rotary vane type VTC system, working fluid pressure is supplied selectively into either one of phase-advance and phase-retard chambers defined in a rotary-vane housing and working fluid pressure is exhausted from the other, in such a manner as to rotate a vane, fixedly connected to a camshaft, in either one of normal-rotational and reverse-rotational directions, thus variably controlling intake valve timing (intake valve open timing and intake valve closure timing) depending on engine operating conditions.
When starting a cold engine, whose coolant temperature is low, an engine crankshaft is rotated by a predetermined crank angle in a reverse-rotational direction for starting the engine with a vane shifted to its maximum phase-advance position. This is because an effective compression ratio becomes high when starting the engine with the vane kept at the maximum phase-advance position, and thus the engine startability can be improved during a cranking period of cold starting operation.
Under a condition where the engine has been warmed up and the coolant temperature becomes adequately high, the vane is shifted to its maximum phase-retard position according to normal cranking operation that the crankshaft is cranked in the normal-rotational direction. This is because an effective compression ratio becomes low when starting the engine with the vane kept at the maximum phase-retard position. That is, by way of such decompression, it is possible to attenuate or reduce noise and vibrations when starting with a warm engine.